Optical transmission systems are generally based on one of two methods of modulation of an optical signal, either direct modulation or external modulation. In the first of these methods, the bias current applied to a laser is modulated, turning the laser “on” and “off”. The disadvantage of this method is that when higher switching speeds are required, the dynamic behavior of the semiconductor material of the laser itself introduces distortion, primarily in the form of chirp. External modulation of an optical signal with a modulating electrical signal produces a modulated optical output signal with significantly reduced chirp, and external modulators have become preferred for high speed applications. In particular, electro-optic modulators such as Mach Zehnder interferometers are typically used for high speed applications.
For many years, external modulators have been made out of electro-optic material, such as lithium niobate. Optical waveguides are formed within the electro-optic material, with metal contact regions disposed on the surface of each waveguide arm. The application of a voltage to a metal contact will modify the refractive index of the waveguide region underneath the contact, thus changing the speed of propagation along the waveguide. By applying the voltage(s) that produce a π phase shift between the two arms, a nonlinear (digital) Mach-Zehnder modulator is formed. In particular, the optical signal is launched into the waveguide, and the 1/0 electrical digital signal input is applied to the contacts (using proper voltage levels, as mentioned above). The optical output is then “modulated” to create an optical 1/0 output signal. A similar result is possible with a linear (analog) optical output signal.
Although this type of external modulator has proven extremely useful, there is an increasing desire to form various optical components, subsystems and systems on silicon-based platforms. It is further desirable to integrate the various electronic components associated with such systems (for example, the input electrical data drive circuit for an electro-optic modulator) with the optical components on the same silicon substrate. Clearly, the use of lithium niobate-based optical devices in a such situation is not an option. Various other conventional electro-optic devices are similarly of a material (such as III-V compounds) that are not directly compatible with a silicon platform.
A significant advance in the ability to provide optical modulation in a silicon-based platform has been made, however, as disclosed in our co-pending application Ser. No. 10/795,748, filed Mar. 8, 2004. FIG. 1 illustrates one exemplary arrangement of a silicon-based modulator device as disclosed in our co-pending application. In this case, a “MOSCAP” structure 1 in terms of a doped (i.e., “metal-like”) silicon layer 2 (usually polysilicon) is disposed over a doped portion of a relatively thin (sub-micron) surface layer 3 of a silicon-on-insulator (SOI) wafer 4, this thin surface layer 3 often being referred to in the art as the “SOI layer”. A thin dielectric layer 5 is located between the doped, “metal”-like” polysilicon layer 2 and the doped SOI layer 3, with the layers disposed so that an overlap is formed, as shown in FIG. 1, to define an active region of the device. Free carriers will accumulate and deplete on either side of dielectric layer 5 as a function of voltages applied to SOI layer 3 (VREF3) and/or polysilicon layer 2 (VREF2). The modulation of the free carrier concentration results in changing the effective refractive index in the active region, thus introducing phase modulation of an optical signal propagating along a waveguide formed along the active region (the waveguide being in the direction perpendicular to the paper).
As of now, such silicon-based electro-optic modulators have been optimized to minimize the optical loss. The optical loss is controlled by reducing optical signal absorption along the extent of the waveguide. Since the absorption is directly related to the carrier doping density, a minimal optical loss requires a minimal dopant density in both polysilicon layer 2 and SOI layer 3. However, this optical loss specification runs in direct opposition to the desire for high speed operation. That is, to provide a high speed (i.e., switching speed greater than 1 Gb/s) device, a relatively high doping density is required. Inasmuch as system requirements are even now moving toward 10 Gb/s, there is a strong need to increase the switching speed of a silicon-based electro-optic modulator, without sacrificing optical power to attain high speed operation.